350 is based on findings that 350 ppm CO2 is the safe limit in the atmosphere for humanity. This is supported by scientists, climate experts, and progressive national governments. There is a conclusion, based on accelerating arctic warming and other early climate impacts, that we are already around 392 ppm. The idea is that if we do not rapidly return to below 350 ppm, that we will reach a point of no return.

Parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere.

There are three numbers, they say, that are important to know when it comes to global warming:

275: For all of human history until about 200 years ago we have been at around 275 ppm. How do we know this? We arrived at this number from data collected by analyzing ice cores. Since the 18th century we have been taking MILLIONS of years worth of carbon, stored beneath the Earth in various forms, and releasing it to the atmosphere.

392: After this relatively short period of time, compared to the age of the Earth, we have reached 392 ppm CO2. This number is rising by 2 ppm every year. This is higher than any other time in recorded history. This is taking into account the regular biogeochemical cycles of Earth. There has been numerous disastrous events related to the amount of CO2 in the atmosphere.

Glaciers are the source of drinking water for hundreds of millions of people. This water is emptying out into the ocean, causing sea levels to rise 6 inches higher than 100 years ago. This is a danger to any city along a coast.

According to the U.S. Geological Survey there is a current drought in the Western part of North America. The United Nations estimates that an area of fertile soil the size of Ukraine is lost every year because of drought, deforestation, and climate instability. There is speculation that the drought in Mekong is a result of climate change.

CO2 and its sister chemical species, HCO3– and CO32- comprise the carbonate buffer system which regulates the pH of seawater. It is becoming more difficult for organisms like corals and clams to build and maintain their shells and skeletons. Coral reefs could start dissolving at an atmospheric CO2 concentration of 450-500 ppm.

The strongest indicator, says 350.org, is the Arctic and the dissapearing sea ice. Sea ice was somewhere below the summer average of 39% for 1979-2000.

Arctic Sea Ice from 1979-2000

350: Some of the world’s leading climate scientists have revised the highest safe level of CO2 to be this number in ppm.